In recent years, optical fiber sensors using an optical fiber where a fiber Bragg grating (FBG) is formed have been used as pressure sensors or strain sensors and the like. Such optical fiber sensors measure pressure or strain amount based on a variation of a Bragg wavelength in response to a deformation of the optical fiber.
As it is publicly known, a Bragg wavelength is determined by a refractive index of an optical fiber and a grating space of diffraction grating. Thus, the Bragg wavelength varies by a variation of the refractive index caused by temperature variation, or expansion and contraction of the optical fiber as well. In other words, under temperature-uncontrolled environment, it is unable to distinguish whether the variation in the Bragg wavelength is caused by pressure or strain, or temperature variation on a measurement object. Consequently, to accurately obtain pressure or strain on the measurement object, temperature compensation for eliminating the variation of the Bragg wavelength caused by temperature variation is required.
As a method for such temperature compensation, for example, an FBG for temperature compensation is arranged to measure only the variation of the Bragg wavelength caused by temperature variation (for example, see patent documents 1 and 2). The FBG for temperature compensation is arranged adjacent to an FBG for measuring such as pressure or strain. A measurement value obtained by the FBG for measurement is corrected based on a measurement value of the FBG for temperature compensation. In this example, the FBG for measurement and the FBG for temperature compensation can be arranged either in series or in parallel.
As another method, a physical structure that suppresses a variation of a Bragg wavelength caused by temperature variation is adopted in an FBG for measuring such as pressure or strain (for example, see patent documents 3 and 4). The patent document 3 discloses a strain gauge adopting a physical structure where a thin part having spring characteristics generated by providing a void part connects two thick parts in a gauge base supporting an optical fiber. The thick parts of the gauge base are fixed on a measurement object in this structure. Expansion of the thick parts with a rise in temperature compresses the both sides of the thin parts. A dimension of each part of the gauge is designed so that the largeness of compression force can cancel the variation of the Bragg wavelength generated in an FBG for measurement. The Patent document 4 discloses a strain sensor where an FBG for measurement and measurement object to which strain is applied are fixed with a temperature compensation member therebetween. The temperature compensation member is made from a material whose coefficient of thermal expansion is a positive/negative reversed value to a coefficient of thermal expansion of an optical fiber.
Further, as another method, a patent document 5 discloses a mechanical sensor where an FBG having uniform grating spaces is adhered and fixed to a tensile member having a part which generates non-uniform strain when tension force is added. In this configuration, when the tension force is added, the grating spaces of the FBG become non-uniform and a bandwidth of reflected wave becomes widen. While, when temperature variation occurs, a Bragg wavelength varies but the bandwidth does not vary. As a result, the mechanical sensor is assumed to be able to measure strain unaffected by temperature variation by measuring the bandwidth variation.